As is well known, various processes and catalysts exist for the homopolymerization or copolymerization of olefins. For many applications it is of primary importance for a polyolefin to have a high weight average molecular weight while having a relatively narrow molecular weight distribution. A high weight average molecular weight, when accompanied by a narrow molecular weight distribution, provides a polyolefin with high strength properties.
Traditional Ziegler-Natta catalysts systems--a transition metal compound cocatalyzed by an aluminum alkyl--are capable of producing polyolefins having a high molecular weight but a broad molecular weight distribution.
More recently a catalyst system has been developed wherein the transition metal compound has two or more cyclopentadienyl ring ligands--such transition metal compound being referred to herein as a "metallocene --which catalyzes the production of olefin monomers to polyolefins. Accordingly, titanocenes and zirconocenes, have been utilized as the transition metal component in such "metallocene" containing catalyst system for the production of polyolefins and ethylene-.alpha.-olefin copolymers. When such metallocenes are cocatalyzed with aluminum alkyl--as is the case with a traditional type Ziegler-Natta catalyst system--the catalytic activity of such metallocene catalyst system is generally too low to be of any commercial interest.
It has since become known that such metallocenes may be cocatalyzed with an alumoxane--rather than an aluminum alkyl--to provide a metallocene catalyst system of high activity for the production of polyolefins.
The zirconocenes, as cocatalyzed or activated with an alumoxane, are commonly more active than their hafnium or titanium analogous for the polymerization of ethylene alone or together with an .alpha.-olefin comonomer. When employed in a non-supported form--i.e., as a homogeneous or soluble catalyst system--to obtain a satisfactory rate of productivity even with the most active zirconocene species typically requires the use of a quantity of alumoxane activator sufficient to provide an aluminum atom to transition metal atom ratio (Al:TM) of at least greater than 1000:1; often greater than 5000:1, and frequently on the order of 10,000:1. Such quantities of alumoxane impart to a polymer produced with such catalyst system an undesirable content of catalyst metal residue, i.e., an undesirable "ash" content (the nonvolatile metal content). In high pressure polymerization procedures using soluble catalyst systems wherein the reactor pressure exceeds about 500 bar only the zirconium or hafnium species of metallocenes may be used. Titanium species of metallocenes are generally unstable at such high pressures unless deposited upon a catalyst support. A wide variety of Group IV B transition metal compounds have been named as possible candidates for an alumoxane cocatalyzed catalyst system. Although bis(cyclopentadienyl) Group IV B transition metal compounds have been the most preferred and heavily investigated for use in alumoxane activated catalyst systems for polyolefin production, suggestions have appeared that mono and tris(cyclopentadienyl) transition metal compounds may also be useful. See, for example U.S. Pat. Nos. 4,522,982; 4,530,914 and 4,701,431. Such mono(cyclopentadienyl) transition metal compounds as have heretofore been suggested as candidates for an alumoxane activated catalyst system are mono(cyclopentadienyl) transition metal trihalides and trialkyls.
More recently, International Publication No. WO 87/03887 describes the use of a composition comprising a transition metal coordinated to at least one cyclopentadienyl and at least one heteroatom ligand as a transition metal component for use in an alumoxane activated catalyst system for .alpha.-olefin polymerization. The composition is broadly defined as a transition metal, preferably of Group IV B of the Periodic Table, which is coordinated with at least one cyclopentadienyl ligand and one to three heteroatom ligands, the balance of the transition metal coordination requirement being satisfied with cyclopentadienyl or hydrocarbyl ligands. Catalyst systems described by this reference are illustrated solely with reference to transition metal compounds which are metallocenes, i.e., bis(cyclopentadienyl) Group IV B transition metal compounds.
Even more recently, at the Third Chemical Congress of North American held in Toronto, Canada in June 1988, John Bercaw reported upon efforts to use a compound of a Group III B transition metal coordinated to a single cyclopentadienyl heteroatom bridged ligand as a catalyst system for the polymerization of olefins. Although some catalytic activity was observed under the conditions employed, the degree of activity and the properties observed in the resulting polymer product were discouraging of a belief that such monocyclopentadienyl transition metal compound could be usefully employed for commercial polymerization processes.
A need still exists for discovering catalyst systems that permit the production of higher molecular weight polyolefins and desirably with a narrow molecular weight distribution. It is further desirable that a catalyst be discovered which will catalyze the polymerization of an .alpha.-olefin monomer(s) to a highly crystalline form of poly-.alpha.-olefin i.e., the product polymer resin is free or substantially free of atactic stereochemical forms of poly-.alpha.-olefin molecules which are amorphous.
Polymers comprised of .alpha.-olefin monomers have hydocarbyl groups pendant from the polymer backbone chain. Relative to the polymer backbone chain, the pendant hydrocarbyl groups may be arranged in different stereochemical configurations which are denominated as, for example, atactic, isotactic, or syndiotactic pendant group configuration.
The degree and type of tacticity of a polyolefin molecule is a critical determinant of the physical properties which a resin composed of such polymer molecules will exhibit. Other critical determinants of the properties which a resin will exhibit are the type and relative concentration of monomers and comonomers, the weight average molecular weight (M.sub.w) of the polymer molecules comprising the resin bulk, the molecular weight distribution (MWD) and the composition distribution of the resin.
Important from a commercial standpoint is the rate or productivity at which a catalyst system will produce a poly-.alpha.-olefin resin of a desired set of properties in terms of tacticity, weight average molecular weight and molecular weight distribution.
The weight average molecular weight (M.sub.w) of a poly-.alpha.-olefin is an important physical property determinant of the practical uses to which such polymer can be put. For end use applications which require high strength and low creep, the M.sub.w of such a resin must generally be in excess of 100,000. Further, for such high strength applications, the poly-.alpha.-olefin resin must generally have a high degree of crystallinity. The degree of crystallinity which a poly-.alpha.-olefin is capable of obtaining is, in major part, determined by the stereochemical regularity of the hydrocarbyl groups which are pendent to the polymer molecule backbone, i.e., the tacticity of the polymer.
Five types of tacticity have been described in poly-.alpha.-olefins: atactic, normal isotactic, isotactic stereoblock, syndiotactic, and hemiisotactic. Although all of these tacticity configurations have been primarily demonstrated in the case of polypropylene, in theory each is equally possible for polymers comprised of any .alpha.-olefin, cyclic olefin or internal olefin.
Atactic poly-.alpha.-olefins are those wherein the hydrocarbyl groups pendent to the polymer molecule backbone assume no regular order in space with reference to the backbone. This random, or atactic, structure is represented by a polymer backbone of alternating methylene and methine carbons, with randomly oriented branches substituting the methine carbons. The methine carbons randomly have R and S configurations, creating adjacent pairs either of like configuration (a "meso" or "m" dyad) or of unlike configuration (a "racemic" or "r" dyad). The atactic form of a polymer contains approximately equal fractions of meso and racemic dyads.
Atactic poly-.alpha.-olefins, particularly atactic polypropylene, are soluble in aliphatic and aromatic solvents at ambient temperature. Since atactic polymers exhibit no regular order or repeating unit configurations in the polymer chain, such atactic polymers are amorphous materials. Since atactic poly-.alpha.-olefins are amorphous, the resins composed thereof have no measurable melting point. Atactic polymers exhibit little if any crystallinity, hence they are generally unsuitable for high strength applications regardless of the weight average molecular weight of the resin.
Isotactic poly-.alpha.-olefins are those wherein the pendent hydrocarbyl groups are ordered in space to the same side or plane of the polymer backbone chain. Using isotactic polypropylene as an example, the isotactic structure is typically described as having the pendent methyl groups attached to the ternary carbon atoms of successive monomeric units on the same side of a hypothetical plane through the carbon backbone chain of the polymer, e.g., the methyl groups are all above or below the plane as shown below. ##STR1## The degree of isotactic regularity may be measured by NMR techniques. Bovey's NMR nomenclature for an isotactic pentad is . . . mmmm . . . with each "m" representing a "meso" dyad or successive methyl groups on the same side in the plane.
In the normal isotactic structure of a poly-.alpha.-olefin, all of the monomer units have the same stereochemical configuration, with the exception of random errors which appear along the polymer. Such random errors almost always appear as isolated inversions of configuration which are corrected in the very next .alpha.-olefin monomer insertion to restore the original R or S configuration of the propagating polymer chain. Single insertions of inverted configuration give rise to rr triads, which distinguish this isotactic structure in its NMR from the isotactic stereoblock form. ##STR2## As is known in the art, any deviation or inversion in the regularity of the structure of the chains lowers the degree of isotacticity and hence the crystallinity of which the polymer is capable. There are two other types of "errors" which have been observed in isotactic polymers prepared using metallocene-alumoxane catalyst systems which act to lower the melting point and/or T.sub.g of the material. These errors, as shown below arise when a monomer is added to the growing polymer chain in a 1, 3 or 2,1 fashion. ##STR3##
Long before anyone had discovered a catalyst system which produced the isotactic stereoblock form of a poly-.alpha.-olefin, the possible existence of a polymer of such micro-structure had been recognized and mechanisms for its formation had been proposed based on conventional Ziegler-Natta mechanisms in Langer, A. W., Lect. Bienn. Polym. Symp. 7th (1974); Ann. N.Y. Acad. Sci. 295, 110-126 (1977). The first example of this form of polypropylene and a catalyst which produces it in a pure form were reported in U.S. Pat. No. 4,522,982. The formation of stereoblock isotactic polymer differs from the formation of the normal isotactic structure in the way that the propagation site reacts to a stereochemical error in the chain. As mentioned above, the normal isotactic chain will return to the original configuration following an error because the stereochemical regulator, the catalytic active metal species and its surrounding ligands, continue to dictate the same stereochemical preference during monomer insertion. In stereoblock propagation, the catalytic active metal site itself changes from one which dictates a monomer insertion of R configuration to one which dictates an S configuration for monomer insertion. The isotactic stereoblock form is shown below. ##STR4## This occurs either because the metal and its ligands change to the opposite stereochemical configuration or because the configuration of the last added monomer, rather than the metal chirality, controls the configuration of the next added monomer. In Ziegler-Natta catalysts, including the above referenced system, the exact structure and dynamic properties of the active site are not well understood, and it is virtually impossible to distinguish between the "site chirality exchange" and "chain end control" mechanisms for the formation of isotactic stereoblock poly-.alpha.-olefins.
Unlike normal isotactic polymers, the lengths of individual blocks of the same configuration in the stereoblock structure vary widely due to changing reaction conditions. Since only the erroneous parts of the chains affect the crystallinity of the resin product, in general, normal isotactic polymers and isotactic stereoblock polymers of long block length (greater than 50 isotactic placements) have similar properties.
Highly isotactic poly-.alpha.-olefins are insoluble in xylene and are capable of exhibiting a high degree of crystallinity and are in part characterizable by their melting point temperature. Accordingly, isotactic poly-.alpha.-olefins are, depending upon their weight average molecular weight exceeding about 100,000, well suited to high strength end use applications.
Syndiotactic poly-.alpha.-olefins are those wherein the hydrocarbyl groups pendent to the polymer molecular backbone alternate sequentially in order from one side or plane to the opposite side or plane relative to the polymer backbone, as shown below. ##STR5## In NMR nomenclature, this pentad is described as . . . rrrr . . . in which each r represents a "racemic" dyad, i.e., successive methyl groups on alternate sides of the plane. The percentage of r dyads in the chain determines the degree of syndiotacticity of the polymer.
Syndiotactic propagation has been studied for over 25 years; however, only a few good syndiospecific catalysts have been discovered, all of which are extremely sensitive to monomer bulkiness. As a result, well-characterized syndiotactic polymers are limited only to polypropylenes. The molecular chain backbone of a syndiotactic polymer can be considered to be a copolymer of olefins with alternating stereochemical configurations. Highly syndiotactic polymers are generally highly crystalline and will frequently have high melting points similar to their isotactic polymorphs.
Like isotactic poly-.alpha.-olefins, syndiotactic poly-.alpha.-olefins are capable of exhibiting a high degree of crystallinity, hence are suitable for high strength applications provided their M.sub.w exceeds about 100,000. Syndiotactic poly-.beta.-olefins are in part characterized by their exhibition of a melting point temperature.
For any of the above described materials the final resin properties and its suitability for particular applications depend on the type of tacticity, the melting point (stereoregularity), the average molecular weight, the molecular weight distribution, the type and level of monomer and comonomer, the sequence distribution, and the presence or absence of head or end group functionality. Accordingly, the catalyst system by which such a stereoregular poly-.alpha.-olefin resin is to be produced should, desirably, be versatile in terms of M.sub.w, MWD, tacticity type and level, and comonomer choice. Further, the catalyst system should be capable of producing these polymers with or without head and/or end group functionality, such as olefinic unsaturation. Still further, such catalyst system must be capable, as a commercially practical constraint, of producing such resins at an acceptable production rate. Most preferably, the catalyst system should be one which, at its productivity rate, provides a resin product which does not require a subsequent treatment to remove catalyst residue to a level which is acceptable for the resin in the end use application desired. Finally, an important feature of a commercial catalyst system is its adaptability to a variety of processes and conditions.
Conventional titanium based Ziegler-Natta catalysts for the preparation of isotactic polymers are well known in the art. These commercial catalysts are well suited for the production of highly crystalline, high molecular weight materials. The systems are, however, limited in terms of molecular weight, molecular weight distribution, and tacticity control. The fact that the conventional catalysts contain several types of active sites further limits their ability to control the composition distribution in copolymerization.
More recently a new method of producing isotactic polymers from an alumoxane cocatalyzed, or activated, metallocene which in its natural state has chirality centered at the transition metal of the metallocene, was reported in Ewen, J. A., J. Amer. Chem. Soc., v. 106, p. 6355 (1984) and Kaminsky, W., et al., Angew. Chem. Int. Ed. Eng.; 24, 507-8 (1985).
Catalysts that produce isotactic polyolefins are also disclosed in U.S. Pat. No. 4,794,096. This patent discloses a chiral, stereorigid metallocene catalyst which is activated by an alumoxane cocatalyst which is reported to polymerize olefins to isotactic polyolefin forms. Alumoxane cocatalyzed metallocene structures which have been reported to polymerize stereoregularly are the ethylene bridged bis-indenyl and bis-tetrahydroindenyl titanium and zirconium (IV) catalyst. Such catalyst systems were synthesized and studied in Wild et al., J. Organomet. Chem. 232, 233-47 (1982), and were later reported in Ewen and Kaminsky et al., mentioned above, to polymerize .beta.-olefins stereoregularly. Further reported in West German Off DE 3443087A1 (1986), but without giving experimental verification, is that the bridge length of such stereorigid metallocenes can vary from a C.sub.1 to C.sub.4 hydrocarbon and the metallocene rings can be simple or bi-cyclic but must be asymmetric.
Metallocene-alumoxane catalyst generally require a high content of alumoxane cocatalyst to be sufficiently productive for commercial use. Accordingly, metallocene-alumoxane produced isotactic poly-.alpha.-olefin resins generally have a higher than desired catalyst residue content. Hafnocene systems, which yield polymers of higher average M.sub.w than the zirconium analogs, have very low activities even at high alumoxane concentrations.
Syndiotactic polyolefins were first disclosed by Natta et al. in U.S. Pat. No. 3,258,455. As reported, Natta obtained syndiotactic polypropylene by using a catalyst prepared from titanium trichloride and diethyl aluminum monochloride. A later patent to Natta et al., U.S. Pat. No. 3,305,538, discloses the use of vanadium triacetylacetonate or halogenated vanadium compounds in combinations with organic aluminum compounds for production of syndiotactic polypropylene.
More recently, a metallocene based catalyst system has been disclosed which is stated to be capable of production of syndiotactic polypropylene of high stereoregularity. U.S. Pat. No. 4,892,851 describes catalyst systems consisting of a bridged metallocene having at least two differently substituted cyclopentadienyl ring ligands which, when cocatalyzed with an alumoxane, is stated to be capable of production of syndiotactic polypropylene. Again, in commercial production to obtain a sufficient productivity level with such catalyst system, the content of alumoxane is undesirably high and consequently the catalyst residue in the resin so produced is undesirably high.
In all methylalumoxane/metallocene catalyst systems the polymer characteristics (M.sub.w, MWD, tacticity type, comonomer incorporation, etc.) are controlled either by modifications to the structure of the metallocene precursor or by adjustment of the process conditions (temperature, pressure, concentrations). In general, adjustment of process conditions does not allow independent control of tacticity level, M.sub.w and comonomer content. Addition of chain transfer agents such as hydrogen gas to the reactor gives lower molecular weight products without affecting tacticity, however, the resulting polymer no longer has unsaturated end groups. End group functionalization is often an important feature in the application of low molecular weight polymers. Given these limitations, one must prepare a wide variety of differently substituted metallocene precursors to access the entire range of desired materials.
In view of the difficulty and practical limitations in the synthesis of bridged metallocene complexes necessary for the production of an alumoxane activated metallocene catalyst system capable of producing crystalline poly-.alpha.-olefins, it would be desirable to develop new catalytic processes which produce highly crystalline forms of poly-.alpha.-olefins of high molecular weight and relatively narrow molecular weight distributions.